The present invention relates to a method of doping a semiconductor layer, a method of manufacturing a thin film semiconductor device, and a thin film semiconductor device, and more particularly, a doping method using a crystallized semiconductor layer by excimer laser anneal, a method of manufacturing a thin film semiconductor device such as a thin film transistor, a thin film semiconductor device in which a semiconductor layer made of a material such as polycrystalline silicon is used as a channel.
With progress of an advanced information age, the importance of input/output devices is increasing rapidly and the devices are in demand to include advanced and sophisticated features. Furthermore, the spread of personal digital assistant machines is remarkable in recent years, and consequently, the technology of producing TFT on a plastic substrate with more excellent weight saving, flexibility, and nondestructive evaluation rather compared with the conventional glass substrates is desired. In such a situation, research and development of active matrix liquid-crystal-display devices (AM-LCD) using a thin film transistor (TFT) and contact type image sensors (CIS) and the like are actively done.
The thin film transistors, in which a semiconductor film made of silicon is used as a channel, can be classified by a material used in order to construct a carrier-transporting layer (active layer), that is, a semiconductor film made of amorphous silicon (a-Si) and a semiconductor film made of polycrystalline silicon having a crystal phase. Polysilicon (poly-Si) or microcrystal silicon (xcexcc-Si) is mainly known as the polycrystalline silicon.
Semiconductors made of the polycrystalline silicon such as polysilicon (poly-Si) or microcrystal silicon (xcexcc-Si) are characterized by the carrier mobility from about 10 to 100 times as high as that of semiconductors made of amorphous silicon, and have very excellent features as a composition material of switching elements. Moreover, the thin film transistors using the polycrystalline silicon for the active layer allow high-speed operation, and therefore are getting most of the attention as the switching elements constituting various logical circuits (for example, a domino logic circuit, a CMOS (Complementary Metal Oxide Semiconductor) transmission gate circuit), multiplexers using these circuits, EPROM (Erasable and Programmable Read Only Memory), EEPROM (Electrically Erasable and Programmable Read Only Memory), CCD (Charge Coupled Device), RAM (Random Access Memory), drive circuits of displays such as a liquid crystal display and an electroluminescent display, and the like in recent years. Moreover, in recent years, remarkable are active matrix type liquid crystal displays employing the thin film transistor (TFT), using such polysilicon for a channel semiconductor film, as the switching element and as a surrounding drive circuit. This is because the constitution of a thin film transistor array, making use of a polysilicon semiconductor film which can be formed at low temperature on a cheap amorphous glass substrate, may allow to implement reflected type panel displays or wide, high-finesse, high-definition, cheap panel displays (for example, a flat type television).
On the other hand, when using poly-Si TFT in switching elements for pixel selection of the liquid crystal display or the like, the OFF state current is high and display quality is low, which is a problem. In MOS transistors using single crystal silicon so far, in a gate reverse bias, a leakage current does not increase, since the channel became in opposite polarity with a source or a drain, a depletion layer is formed and enough pressure-proofing and rectification property can be shown. However, with the poly-Si TFT, a problem arises that a high leakage current occurs since electric current flows through the grain boundary of crystalline particles composing the semiconductor film or through the defect of the particles themselves. Furthermore, since the MOS transistors are not used under very high gate reverse bias, the leakage current has not become a problem. However, in the poly-Si TFT, for example used for the active matrix type liquid crystal displays, the leakage current poses a big problem since it is used under the reverse bias of about 10 V or more. Such a problem is especially important when the poly-Si is used for the thin film transistor for pixel selection of the liquid crystal displays.
In order to reduce the leakage current, it is effective to relax the electric field in the drain edge, and it has been known that LDD (Lightly Doped Drain) structure is effective (General Conference of The Institute of Electronics and Communication Engineers, 2-20, pp. 271, 1978). The structure forms the region which activated the impurities under a low dose such as 1xc3x971014/cm2 or less in the edge part of the drain region, and relaxes the electric field in the edge part of the drain region.
The thin film transistor having the LDD structure is formed, for example, by the following processes so far. First, as shown in FIGS. 5A to 5C, an amorphous silicon containing hydrogen (a-Si:H) film is formed on a glass substrate 101, and dehydrogenation is performed by the lamp anneal. Then, a polysilicon (poly-Si) semiconductor film 102 is formed by crystallizing the amorphous silicon film using laser irradiation. Then, a gate insulating film 103 and a gate electrode 104 are formed, and heavy doping of impurity ions is performed by using the gate electrode 104 as a mask (FIG. 5A), where the gate electrode 104 has already been patterned to cover a channel region and an LDD region. Subsequently, the gate electrode 104 is again patterned to cover only the channel region. And light doping of impurity ions is performed by using the re-patterned gate electrode 104 as a mask. Consequently, source drain regions 105a and 105a are formed to have the LDD structure with low concentration impurities regions 105b and 105b formed on the sides of the channel region. Then, an interlayer insulating film 106, contact holes 106a, and wiring layers 107 are formed, and the wiring layers 107 are connected to the source drain regions 105a and 105a through the contact holes 106a. More particularly, such processes have been disclosed in Japanese Unexamined Patent Application No. 2000-228526.
When forming the thin film transistor having the LDD structure by such a method, there is a problem of the difference or the variation in lengths of the LDDs on both sides of the channel region (thicknesses of the LDD regions between the channel region and contact regions) due to deviation of the mask during patterning of the gate electrode 104, and the like. This causes other problems that the properties of the thin film transistor vary and the productivity of the thin film transistor decrease. Moreover, the LDD lengths should not be set to about 2 xcexcm or less in order to secure a mask alignment margin. For this reason, the resistance of the low concentration impurities regions 105b and 105b performing as the LDD regions becomes high, and the carrier mobility decreases, which is a problem. Therefore, in a self-alignment type process where the controllability of the LDD lengths is good, it is important to develop a certain process where the controllability is enough at a low dose such as 1xc3x971014/cm2 or less.
By the way, as for the poly-Si TFT, the highest process temperature reaches about 1000xc2x0 C. in the manufacturing process. Therefore, silica glasses or the like having an excellent heat-resistant property are used as an insulating substrate for the poly-Si TFT manufacturing. That is, it can be difficult in the manufacturing process to use a glass substrate with a comparatively low melting point. However, for a cost reduction of the liquid crystal displays, the use of the glass plate materials with a low melting point is indispensable. Then, in recent years, the development of the so-called low temperature process with the highest process temperature reaching 600xc2x0 C. or below is making progress, and the production of such devices is practically done. Furthermore, recently, using a plastic plate to easily form a larger area under lower temperature has been also examined. The deformation temperature of the plastic plate is at most 200xc2x0 C., even when formed from a heat-resistant material. Therefore, when the substrate is formed from the plastic, all processes must be performed on the condition of super low temperature as compared with the conventional conditions, that is, at 200xc2x0 C. or below.
With the larger type of liquid crystal display, in the low temperature process for the poly-Si TFT, the ion doping and the plasma doping, which allow doping impurities into the semiconductor thin film with a large area with a fine throughput, are used. The ion doping is the method of ionizing an impurity gas and then irradiating the impurity ions all at once onto the large area semiconductor thin film by accelerating electric field without performing a mass separation. The plasma doping is the method of ionizing an impurity gas and a deposition gas simultaneously, and deposit including the impurity ions on the substrate surface. On the other hand, ion implantation is the method of performing the mass separation of impurity ions, producing an ion beam of the separated ions and irradiating the ion beam onto the semiconductor thin film. Although the ion doping and the plasma doping are advantageous to the formation of the larger area type, these processes pose problems that the film can contain hydrogen in large quantities which can blow off and break the film at the time of crystallization by the excimer laser (ELA: Excimer Laser Anneal), and that it is difficult to perform the lower temperature process using the plastic plate or the like at the required temperature for dehydrogenation (400xc2x0 C.). Moreover, there is also a problem that these methods are not suitable for the self-alignment type process in principle.
By the way, the Laser-Induced Melting of Predeposited Impurity Doping (LIMPID) attracts attention recently as being a method in which doping can be done in a process at 200xc2x0 C. or below. The LIMPID is the method of ionizing an impurity gas, adsorbing the impurity ions on the semiconductor thin film surface, and melting the ions into the film with an excimer laser, and attracts attention not only because the hydrogen cannot be entrapped into the film, but also because it is most appropriate to the self-alignment process as well as to the low temperature process (refer to Japanese Unexamined Patent Application No. SHO 61-138131, Japanese Unexamined Patent Application No. SHO 62-002531, Japanese Unexamined Patent Application No. SHO 62-264619, and Japanese Unexamined Patent Application No. HEI 9-293878).
With the LIMPID, the high dose such as from about 1xc3x971015 to 1xc3x971016/cm2 of the impurities can be electrically activated in the semiconductor thin film. However, in principle, it is difficult to precisely control the dose of 1xc3x971014/cm2 or less of the impurities. Because the high dose of from about 1xc3x971015 to 1xc3x971016/cm2 of the impurities is activated by the excimer laser anneal, even when, for example, the impurity ions of an atomic layer are adsorbed on the top of a Si surface. Furthermore, since the adsorption of the impurity ions of the atomic layer occurs for an extremely short time in the conventional methods, the control at the low dose is difficult.
On the other hand, the conventional ion implantation is the most appropriate to the self-alignment process and enables also the control at the low dose. Since the temperature of the substrate generally increases in the process for the silicon substrate, the method of attaching a cooling plate by the electrostatic chuck of the substrate and radiating heat from the back side is taken in the process. However, it is difficult to apply such a method to the plastic plate considering the thermal conductivity and electrical conductivity of the plastic plate. Moreover, there are other problems that the impurities cannot be implanted into the semiconductor thin film with the large area all at once, and that the throughput gets worse in the manufacturing the large-sized liquid crystal displays.
Moreover, in the crystallization process with the laser, since irradiation time is about 30 ns which is extremely short, solid phase diffusion cannot occur, but only liquid phase diffusion can occur. In such a case, a steep junction is formed at the boundary between the channel and the source drain region. Therefore, the problems of grain boundary leak and hot electrons are remarkable compared with the processes using the furnace anneal and lamp anneal in which solid phase diffusion can occur. Therefore, when a laser activation process like the process for the low heat-resistant substrate is required, it becomes indispensable to form the LDD structure under excellent control.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide a method of doping a semiconductor layer which can form a lower concentration impurity diffusion region under excellent control even when a low heat-resistant substrate is used, a method of manufacturing a thin film semiconductor device, and a thin film semiconductor device.
A method of doping a semiconductor layer according to the invention comprises the steps of forming an energy beam permeable mask on a part of a surface of a semiconductor layer, adsorbing dopant ions on a surface of the semiconductor layers except a region in which the mask is formed, and introducing the dopant ions into the semiconductor layer with irradiation of an energy beam onto the semiconductor layer having the formed mask.
According to the method of doping the semiconductor layer of the invention, while the dopant ions adsorbed on the surface of the semiconductor layer diffuse in the semiconductor layer by the irradiation of the energy beam, the mask is constituted so that the energy beam may pass through, and therefore the region where the energy beam is irradiated through the mask as well as the region where the energy beam is directly irradiated is fused by the irradiation of the energy beam. Therefore, diffusion of the impurities in transverse direction arises from the surfaces of the regions other than the mask, but the impurities concentration in the mask area become smaller, since the area is more separated from the sources of the diffusion of the adsorbed ions compared with the regions where the energy beam is directly irradiated, and a lower concentration impurity diffusion region can be formed in the semiconductor layer with sufficient accuracy.
Moreover, a method of manufacturing a thin film semiconductor device of the invention comprises the steps of forming a convex part on a surface of a semiconductor layer along with an insulating film in between, forming an energy beam permeable mask around the convex part, adsorbing dopant ions on a surface of the semiconductor layer except a region in which the mask is formed, and introducing the dopant ions into the semiconductor layer with irradiation of an energy beam onto the semiconductor layer having the formed mask.
Other and further objects, features and advantages of the invention will appear more fully from the following description.